Not receiving texts on iphone?

I just recently got an iPhone 4s on sprint and everything is working fine. I just started talking to this person and he's been calling and asking why I'm not answering his texts. Thing is I'm not recievig any but I can get
them from other people. I'm not sure how to fix this

Added (1). Already registered, just this one person I'm having a problem with

You haven't registered for messaging service. Call your local Sprint company to get it started.

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